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Not that this would be ecologically feasible but what if you dug a tunnel from the pacific ocean to death valley (-300 feet). Then you could get some power out of the potential water drop. Then as the water floods the valley it's so hot it would evaporate and you could keep letting the water in. The evaporated water would rain on the next mountain down wind and create arable land.

Siphons have a maximum theoretical height limit relating to the minimum pressure inside at the peak. Simply put when the pressure gets low enough to boil water at ambient temperature then the siphon no longer works.

Interesting, but what's the advantage of this over condensing the vapour from the pools directly?

It is orders of magnitude less expensive. If you want to condense the vapor, you first need to collect the vapor. Evaporation ponds typically cover thousands of hectares. Building an enclosure to collect the vapor over such an area would cost megabucks or even gigabucks. Once you collect the vapor you need to compress and cool it to turn it into liquid water. This is also very expensive.

If simply evaporating seawater was a cost effective way to produce fresh water, the world would have not water shorta

* Step 1: Concentrate brine in large evaporation ponds
* Step 2: Generate electricity from the osmotic difference between this brine and normal seawater
* Step 3: Use the electricity to split seawater into fresh water and brine
* Step 4: Recycle the brine back into the evaporation ponds
* Step 5: Profit!

The reason this works is that you are effectively collecting the solar energy that shines on the evaporation ponds.

From the article:
"In fact, the fresh water doesn't have to come from a river. Cui says that storm runoff, gray water, or even treated sewage water could potentially be used. As an added benefit, the mixing entropy process can be reversed to produce drinking water by removing salt from ocean water."

Making electricity from the difference in salinity (the amount of salt) in fresh water and sea water is not a new concept. We've previously covered salinity power technology [gizmag.com], and Norway's Statkraft [gizmag.com] has built a working prototype power plant. But the Stanford team, led by associate professor of materials science and engineering Yi Cui, believes their method is more efficient, and can be built more cheaply.

TFA explicitly mentions the Statkraft project. However, there seems to be a significant difference between the two; where statkraft is using the salinity to create pressure and power a conventional turbine generator, this article is about creating current directly, which should theoretically improve efficency a lot.

According to TFA a, 50cubic meter/second flow of fresh water could yield up to 100MW.

You do know that there is an amazingly simple way to separate the salt from the water, right? It is called evaporation.

The concepts of desalination are certainly quite simple, its the economics that are complicated.

He means natural evaporation. Oceans are under the Sun all the time. You can't "use up" fresh water by intercepting water that's about to fall on the ocean and making it into salt water. That is happening all the time anyway, those rivers are constantly dumping fresh water into the ocean. Luckily, water evaporates off the oceans and comes back down as fresh water as rain, filling up those rivers and continuing the cycle.

This certainly wouldn't be appropriate everywhere. But, consider the Mississipi River Delta. It's dumping massive amounts of fresh water into salt water anyhow.

It hardly seems like diverting a small percentage of the fresh water being dumped into the ocean by nature, extracting power from it as it gets salty, then dumping the brackish water from the power plant into the ocean, in any reasonable way reduces the fresh water supply, does it?

Or, do you propose that we completely dam up the Mississippi so we don

It looks like a potato battery (that we used to make little clock kits back in the 80s) or any galvanic battery dating back 100+ years, but with a tweak to get more out of it, implemented on a larger scale, and slapped with a "New and Improved, now with NANOTECHNOLOGY" sticker.

In Florida most drinking water is obtained from wells. Deep wells tend to be brackish and require desalination of the water to be usable. It would seem then that a combination use of waste water and deep well water would work. Also the battery sounds like it acts as a desalination device during discharge so it might serve the purpose of both desalination and power generation.

"The Stanford scientists are currently working on modifications to get the battery ready for commercial production. For example, the silver electrode is very expensive, and they hope to develop a cheaper alternative."

I'm really at a loss on this. How expensive can a silver electrode be, if you're producing enough power to charge for it? Silver while pricey (currently ~ $39.00/oz) It's just a tad more expensive than Lithium (currently ~ $31.50/oz) and if this thing really worked. they'd pay

Normally, batteries work by leaching material from one electrode into the water, while precipitating ions on the other. By draining the battery, you actually "consume" one of the electrodes. Recharging work if the process can be reversed.

However, if the electrolyte is changed between charging and decharging, effectively the manganese dioxide or silver ions dissolved are now gone, which has two effects:

In learning about thermodynamics I had learned that, where there's a gradient, you can extract energy, be it a gradient of temperature, electrical field... or even chemical concentration. But it's one thing to know it's theoretically possible, and another thing to actually pull it off in a way that extracts meaningful energy. Good work, scientists and engineers.

You comment is funny for me. Just a bit ago I posted a comment about how every single person who brings up thermodynamics on Slashdot doesn't know what it means. And then here I find, possibly the first, comment about thermodynamics on Slashdot with a an actual clue.

Glad to hear it! But I didn't know I was saying something all that insightful or demonstrative of thermodynamics understanding.

It is kind of counterintuitive that you'd be able to extract energy just because there's a difference in concentration between two bodies, but it makes more sense once you've read about the Gibbs paradox (esp. Jaynes's handling) and how you can power a mechanical device by using membranes that differ in their permeability to the different constituents of the mixture.

So, 13,000 gallons per second of fresh water flow and we can get around 100MW. Let's go on a math exercise, shall we?

The average combined cycle plant is (at a minimum) around 400MW. Not including co-gens, etc. Just normal power plants sitting out in the middle of nowhere. Fukishima is around 4900MW. Fukishima isn't really fair because it is, by any measure, a large nuke plant. But, 400-1200MW is not an unreasonable range for "typical" power plants in the US, regardless of the technology used (coal, nuke, combined cycle, direct fire, etc)

At 400MW, you are talking 52,000 gallons PER SECOND of water flow. That, by any measure, is a shitload of flow. At 1200MW, we are talking 156,000 gallons per second.

For comparison, I just looked up the flow rate of the Mississippi river at the high water dam near Lake Itasca. Going thru the Upper St Anthony's falls lock and dam, the flow rate is around 90,000 gal/sec [nps.gov].

So for ONE reasonably sized power plant, you would need fresh water flow that is the equivalent of the Mississippi River.

Uh, you are looking through the telescope the wrong way. That flow you quoted is at the UPPER end. From the same site you linked to, the flow is 50 times greater at the Delta, where you find the OTHER side of the cell, salt water.

For a large-ish 2000MW plant, it would need 260000 gallons per second of fresh water. At the MOUTH on the Mississippi (where it meets the ocean), that would make up about 5.8% of the over all flow of the river. Not as big of a deal as you are suggesting

If you diverted a quarter of the flow (probably possible, if not practical), you could supply a plant that provides over 8000MW. It would certainly be a big facility, but that's also a lot of electricity.

In fact, the fresh water doesn't have to come from a river. Cui says that storm runoff, gray water, or even treated sewage water could potentially be used. As an added benefit, the mixing entropy process can be reversed to produce drinking water by removing salt from ocean water.

Wait, you actually think water is disappearing, going poof? Where do you think this water is going? Water is not something you use up and then there is no more. You see, it evaporates, and then it rains down again clean. Now it may not be where you expected it would be, or it may end up unfit for use in areas with contaminants, but the water is still there.

You realize there are nearly inexhaustible supplies under the ground right? If you suck it out faster than it seeps back down, guess what, the water still exists. We could potentially use it faster than we harvest it, but to assert that water is a scarce resource is very, very misleading. You can always expand your collection techniques.

Or are you suggesting we are in danger of locking up *all* of the hydrogen and oxygen on the earth in to other compounds?

Oh, you know that salt water? Let it evaporate, and magically you have more fresh water.:P

They could easily have free/nearly so fresh water for everyone, but those areas are generally run by less forward thinking leaders. Recently there has been some move to change that, but seems to have tapered off.

The same way roads and all other government projects are funded.Excluding the UAE, most of the middle east could easily fund such projects with a tax on exported oil. Not all UAE nations export enough oil, so they could then have their various sheiks pay for it. Desalination is a well understood technology.

Truthfully though, these nations have other problems perhaps even more pressing. By that I mean the political issues that have created the current situation to begin with.

I think he's talking about taking water and sending it to Arizona where it then evaporates in the desert and doesn't actually make it to the end of the river. I'm guessing of course. But as for these "inexhaustible" supplies under ground, you should read about the supply in the midwest [wikipedia.org] which requires drilling to new depths because it is being depleted. Should you think going deeper is always an option, you may want to read the recent stuff of fracking to see how the deeper water is being deliberately contaminated. There are solutions to these, problems, but what we are doing vs what we could be doing don't really match.

Wait, you actually think water is disappearing, going poof? Where do you think this water is going?

You might want to Google "photosynthesis". Major rivers no longer reach the ocean because we've diverted them for use in industrial agriculture. And yes, that water really does cease to exist as water.

Of course, realistically, most of it ends up going to waste, either soaking into the ground or evaporating; Yes, we can theoretically reduce those losses drastically, but as it stands, for both human consu

Ground water isn't a closed system in any sense. If water is taken from an aquifer at a rate greater than it is replenished then the level of the aquifer will fall and even temporarily dry up until the water levels can replenish, this could take a months, years or even centuries dependent on local geography and climate.

In many places, aquifer depletion is a major engineering obstacle necessitating boreholes to be drilled ever deeper to maintain their rate of water extraction until the point they are simply

I said you can extract if faster than it is replinished, but it really is inexhaustable. You will not run out. Once the reserve is gone, you can only get it as fast as it seeps down, but it isn't *gone*

The point of the post was that people are far too accepting of the concept that *everything* is scarce. Water just isn't one of those things.

The planet will not run out of water. It just isn't going to happen. Sure, we might have to move usage around a bit in some cases, but the idea that water will magi

If you've ever had tap water from a town along the Mississippi River, you're probably drinking some thousands of other humans' piss. Possibly even you own, if you drive south along the river. Towns on big rivers don't drill for water. They treat the river water for drinking, then treat the sewage and release it back into the river.

I would gladly trade this for the aging coal power plant that currently sits on the banks of my local estuary. I am inclined to believe that this will be better for my local environment than the coal burning.

Because river deltas and estuaries are sensitive environments, the Stanford team designed their battery to have minimal ecological impact. The system would detour some of a river's flow to produce power, before returning the water to the ocean. The discharge water would be a mix of river water and sea water, and released into an area where the two waters already meet.

I have never met a single person that identified themselves as an environmentalist that actually was one. Every single person I have met that actually was an environmentalist would not self identify as one.